A switching mode power supply (SMPS) converts power from a source, for instance Vdd, on an integrated circuit chip into a voltage or current to be used to power a portion of the circuits on that integrated circuit chip. Switching mode power supplies comprise buck, boost and buck-boost power converters. The buck converter stores energy into an inductor and provides an output voltage that is a result of the reluctance of the inductor to change current flowing in the inductor. The boost converter stores energy into an inductor and provides an output greater than source voltage and the buck-boost converter produces an output that is either higher or lower than the source voltage.
The buck power converter generates a pulse width modulated (PWM) switching voltage at the LX node shown in FIG. 1, which is then filtered by an inductor L1. Generally, buck converters operate in one of two modes, PWM mode at a fixed frequency, or pulse frequency modulation (PFM) mode where the frequency is allowed to change with load current.
Typically the PFM mode is used for low-power operation and can be highly efficient. In the PFM mode, the PMOS transistor P1 is typically turned on when the output voltage falls below a low threshold. The PMOS transistor is then turned off when the current in the inductor rises above a fixed limit, or if the output voltage rises above an upper threshold. When the PMOS transistor P1 is turned off the current in the inductor L1 continues to flow, until the inductor is discharged. This current must be supplied from ground. This can be done using a diode, but the voltage drop across the diode reduces the efficiency of the buck converter. Therefore, most high efficiency buck converters use an NMOS transistor N1, directly controlled by the buck control circuitry, and when the PMOS transistor P1 turns off, the NMOS transistor N1 is turned on.
If the load is low, the buck may only need to switch to at a low frequency to supply the output current. If the NMOS transistor is left on until the PMOS transistor is triggered again, the current in the inductor will go negative, the NMOS transistor will end up discharging the output of the buck converter and power will be wasted. Instead the NMOS transistor is turned off once the current in the inductor reaches zero current, which is typically referred to as an active diode behavior. In one common implementation of a buck converter, the active diode function is implemented by measuring the voltage across the NMOS transistor. If the voltage at the LX node is negative, the current is still positive, and the NMOS transistor is kept on. But once the voltage at the LX node goes above ground, the NMOS transistor is turned off.
US 2006/0279970 A1 (Kernahan) is directed to control system and method for simultaneously regulating the operation of a plurality of different types of switching power regulators including not having the regulator feeding current back to the supply. U.S. Pat. No. 8,222,879 B2 (Nguyan) is directed to a circuit that includes a buck voltage regulator couple to an active current modulator, which is operative to detect negative current in the low side switch of the voltage regulator. In U.S. Pat. No. 7,443,699 B2 (Lhermite) is directed to a power supply controller that uses a negative current of a power transistor to detect a point for enabling the power transistor when driving an inductor. In U.S. Pat. No. 7,365,661 B2 (Thomas) a control system and method is directed to simultaneously regulating the operation of a plurality of different types of switching power converters. In U.S. Pat. No. 7,095,220 B2 (Kernahan) a method is directed to controlling an operation of a switching power converter which includes a first and second series connected transistors and including the handling of “negative” current flow. U.S. Pat. No. 6,911,809 B2 (Kernahan) is directed to a controller configured to control the pulse widths of a plurality of pulse width modulated switching power supplies, wherein a discontinuous operation, current is not fed back to the supply from the inductor.